1. Field of the Invention
The present invention relates to an inspection apparatus for a sensor element for use in a gas sensor, and particularly to an inspection apparatus for inspecting element characteristics in a gas atmosphere.
2. Description of the Background Art
Conventionally, various gas sensors have been used for recognizing a concentration of a desired gas component in a measurement gas. For example, as a device for measuring a NOx concentration in a measurement gas such as a combustion gas, known is a NOx sensor having a sensor element which is formed using an oxygen-ion conductive solid electrolyte such as zirconia (ZrO2) (for example, see Japanese Patent Application Laid-Open No. 2006-284223 and Japanese Patent No. 3537983).
In sensor elements of gas sensors including the NOx sensors disclosed in Japanese Patent Application Laid-Open No. 2006-284223 and Japanese Patent No. 3537983, a concentration of an object component in a measurement object gas is obtained by utilizing the fact that in a case where the measurement object gas component is decomposed at a measuring electrode, the amount of oxygen ion occurring at that time is proportional to a current flowing in the measuring electrode and a reference electrode. To be specific, a concentration value of the object component is recognized as follows: a relationship (sensitivity characteristics, a concentration profile) between a concentration value and a current value (output signal value) in each individual sensor element is obtained in advance by using a mixed gas whose object component concentration is already known; and in an actual use, a measured current value is converted into a concentration value based on the sensitivity characteristics.
The above-described sensitivity characteristics are determined before each gas sensor is used (for example, before shipment), and normally dealt as fixed characteristics without being changed when the gas sensor is used afterward. This is based on the assumption that actual sensitivity characteristics do not vary during the use of the gas sensor. If the actual sensitivity characteristics change over time, the concentration value which is calculated based on the sensitivity characteristics determined at the time of shipment loses its reliability as the use of the gas sensor continues, and eventually the gas sensor loses a measurement accuracy set in its specification.
To ensure the measurement accuracy, in a manufacturing process of a sensor element, an element characteristics inspection is performed to evaluate electrical characteristics of a sensor element in an atmosphere of a mixed gas similar to a measurement gas, and if sensitivity characteristics of a sensor element vary or do not satisfy a predetermined specification (management range), it is determined that the sensor element is a defective product.
Since the sensitivity characteristics slightly differ among individual sensor elements, it is necessary to perform the element characteristics inspection on all the individual sensor elements, in order that all of gas sensors shipped as product satisfy a predetermined quality (measurement accuracy). Moreover, from the viewpoint of reliability, the element characteristics inspection has to be performed on all the sensor elements under substantially the same conditions. On the other hand, for improvement of productivity, it is demanded to shorten an inspection time by concurrently inspecting a plurality of sensor elements.